It has been observed that when certain finely divided or powdery polymeric materials are attempted to be admixed with a viscous liquid, such as with a liquid prepolymeric medium, bubbles are formed because of air entrainment during the mixing. Another major problem caused by entrained air is increased slurry viscosity, which is a limitation to the amount of powder additive which can be employed. As a result pockets may be formed in the finished polymer made from the mixture, leading to the presence of voids and holes. Apparently the molecules of air interfere with the wetting of the powder particles and are not readily disassociated from such particles. The extent of this problem is dependent upon the amount of air trapped during the mixing, the uniformity of the entrapment and the size of the individual bubbles. In order to obtain acceptable performance in a system, this entrained air must be eliminated or controlled, depending upon the nature of the polymeric system involved. For example, the presence of numerous small bubbles uniformly dispersed throughout the polyol slurry precursor of a polyurethane or other polymer foam may be acceptable. It is not acceptable, however, if the bubbles are large and/or localized, which will lead to large holes in the finished foam.
The solutions to the problems presented vary with the polymeric system employed. In the case of thermoplastic polymers, the air is mechanically removed via shear intensive equipment (such as extruders, opposed roller mills, etc.). These thermoplastic polymers generally are already polymerized prior to the addition of the solid material (such as fillers for example) and these have the ability to build up some internal shear of their own to combine with the mechanical shear for driving the air out of the system. Such is the case, for example, in the addition of talc as a filler in polypropylene. Systems of this sort present no real problem.
However, in the case of thermosetting polymers, mixing of solids is generally carried out in the precursor system or otherwise where further polymerization is yet to occur and wherein a different type of less intensive processing equipment is employed. There is but a minimal internal shear generated when the finely divided solid material is added to the liquid polymerizing component rather than to the final polymerized product. The turbulence in such liquid components will usually increase the amount of air incorporated in the liquid. Among problem materials in this category are resin components for the production of phenolic, unsaturated polyester, epoxy and polyurethane resins.
None of the solutions to the indicated problem that have been proposed have been found entirely effective or acceptable. For example, it has been proposed in the case of unsaturated polyesters, to incorporate certain additives to decrease the viscosity buildup due to entrapped air. When the polymer system is pressed at the lower viscosity most of the air can be forced out of the resin system before completion of cure. This approach attempts to minimize the result rather than eliminate the cause, and in the interim increases the expense. Only certain solid materials can be incorporated into this type of resin system, and the proposed solution is limited to the special problems of this particular resin system.
A number of different techniques have been employed in polyurethane resin systems but none of these have been found to solve the bubble formation problem. Agitation of the components, for example, is minimized to keep down the amount of additional air introduced by agitation. Then the samples are placed in a vacuum over to dry and draw off some of the incorporated air. However, this approach is very time consuming and requires vacuum equipment not generally used by this industry.